Main battery box, originally designed to accommodate four 6-volt golf-cart batteries, but shown here with 1 group 27 battery. In the upper-right, you can see the separate battery box for the dedicated engine start battery (at the time another group 27).
Here is the box with it's lid on:
It is located right under the companionway ladder
Here two shots of the separate box for the engine start battery, located right next to the starter:
I never did get around to the four 6-volt golf-cart batteries, instead last fall I replaced the single Group 27 with three large Odyssey batteries, for a total of a 300 amp-hour bank:
(FWIW, there is now a box enclosing the engine space, not apparent in these photos)
edit: with the Odyssey batteries (sealed AGM type), I don't strictly need an acid-proof vented box to hold them, but regardless, batteries are heavy (unless you've got LiFePO4 batteries) and need to be well secured to prevent them moving around or coming loose in a knock down.